Use Airport Extreme instead of Xfinity Combo

Will be upgrading to Yosemite shortly.

So far, Yosemite is Apple's worst operating system ever as far as network bugs, so you might want to hold off awhile longer.

I would like to connect the AIrport Extreme back to my network and only use that for wifi. I basically want to bypass the WIFI on the Xfinity device and only use the Cable Modem part of this. I know how to hook up but when I logged into the Cable Modem there was no option to shut off WIFI.

That is very unusual, but more and more ISPs are providing modem/routers with a very limited feature set, so it is possible that you may have a product that will not allow the wireless function to be turned off.

Did you ask Xfinity support about this?

If I keep both active won't my signal be split.

Not if you don't use the Xfinity wireless and just use the AirPort wireless. If it really is not possible to turn off the wireless function of the Xfinity modem/router, then it would be good idea to keep the Xfinity and Airport separated by 5-6 feet, or a few meters, to minimize the chances of wireless interference occurring between the two devices.

ethernet the extreme and put it in bridge mode using Airport Utility. Create a wireless network also.